Normally propellers of surface piercing propeller type are arranged and mounted so that the propeller, upon driving the boat in a water surface planing position, operates with at least 50% of the driving surface above the sea water surface or otherwise in a corresponding gas or air flow. Propellers of this type exhibit many advantages over conventional propeller arrangements, where the propeller as a complete operates under water. Among other things, the fact that the propeller in part operates in the air involves the advantage that there will be very little, if any, cavitation, and that the propeller, in spite exhibiting very strong propulsion force, is subjected to very small, if any, cavitation damages.
In some respects, however, a surface piercing propeller also can give rise to some problems. Since the propeller blades to a great part of their revolution operates in air there will appear a lifting force when the propeller, after having rotated in the air, hits the water surface. The propeller blades thereby hit the water surface with a substantially horizontal force. Said lifting force can amount even up to 30% of the propulsion force of the propeller, and this will reduce the effective propulsion ability of the propeller.
Since only half the propeller operates under water the impact force of the propeller against the water surface also will make the propeller shaft be subjected to a certain unevenly distributed load which may cause problems with wear of bearings, vibrations, and in the worst case, rupture of the propeller shaft.
There has been a desire to make it possible to make use of the favourable properties of surface piercing propellers by using such surface piercing propellers also for propulsing ships in the cases where the propeller, even at full speed of the ship, operates entirely under water. This, however, has not met with success depending on a problem which has been difficult to solve:
Since the propeller operates entirely under the water surface there will be a strong sub pressure against the surface of the propeller facing forwardly and this leads to cavitation and cavitation damages depending therefrom. Cavitation damages can generally be compared with corrosion damages. For eliminating such problem it is necessary that the surface piercing propellers are ventilated, so that the sub pressure against front surfaces of the propeller blades can be unloaded at each individual revolution of the propeller, and whereby air bubbles, which tend to appear at the (front) suction side of the propeller in case of cavitation, are removed. The ventilation also should be such that the propeller blades, during the greatest part of their revolution, for instance up to 90% of their revolution, are allowed to operate in the water, whereas said propeller blades, during a very little part is its revolution, operate in the air. In this last mentioned case the sub pressure on the suction side of the propeller blade is unloaded, and this strongly reduces the risk of appearance of cavitation.
The ventilation of a surface piercing propeller, which operates entirely under water has, in turn, caused special problems.